Apparatus for corona treatment of polymer film including a flexible metal foil electrode



H. G. SCHIRMER CORONA TREATMENT OF INCLUDING A FLEXIBL Filed Au3,346,480 POLYMER FILM E METAL FOIL ELECTRODE g. 26, 1964 Oct. 10, 1967APPARATUS FOR Fig.1

22 CORONA DISCHARGE Fig.2

United States Patent Office assignor to W. R. New York, N.Y., acorporation of This invention relates to apparatus for the coronatreatment of film.

It has previously been proposed to improve the printability ofpolyolefins, such as polyethylene, by subjecting said films to highvoltage electrical discharge. In proposed prior processes the film ispassed between a pair of electrodes having an extended surface area towhich is ap plied a high value alternating potential sufficient toproduce a diffuse corona between the electrodes. The corona isapparently caused by partial breakdown or ionization of the atmospherearound an electrode. The electrodes must be so spaced that the filmsurface is exposed to the corona. The electrodes may comprise a pair offiat plates positioned parallel to one another. The electrodes may alsocomprise a drum having a stator spaced apart and concentric therewith.There may also be positioned between the electrodes a sheet ofdielectric material to prevent an arc-over and damage to the film beingtreated in the event that said film has pin holes or other weak spotstherein. The dielectric covering also prevents pitting of the electrodeand helps to spread the corona over the entire width of the electrodeand causes the film to be a minor portion of the total dielectric in thegap. Suitable dielectrics for ground roll coverings are glass, Mylar,epoxy resins and elastomers, such as chlorosulfonated polyethylene,silicone rubber and the like, and anodized coatings. The elastomers aregenerally preferred since the only maintenance required is that it bekept free of any surface irregularities to prevent treat-through.

One of the difliculties with a blade or area type electrode has beenachieving a uniform corona over the entire film surface. Generally, itis believed that the electrical discharge is effected at sharp edges. Ifthe electrode has points and peaks which are closer to the dischargedelectrodes, the discharge will preferentially take place at these pointsresulting in spotty corona treatment.

It is an object of the invention to provide apparatus for the coronatreatment of film.

It is another object of the invention to provide apparatus for theproduction of an electrical discharge.

Still another object of the invention is to provide apparatus for theformation of a corona.

These and other objects of the invention will be readily apparent tothose skilled in the art from the accompanying disclosure, drawings andappended claims.

It has now been surprisingly found that a thin, flexible, electricallyconductive, area electrode, for example, aluminum foil, in conjunctionwith a discharging electrode will produce a uniform electrical dischargeand corona if a high-value alternating potential is applied across saidelectrodes. Further, if the flexible electrode is minimumly supported sothat the remainder of the electrode is unsupported, said electrode willadjust itself to a coronaproducing distance from the dischargingelectrode.

In another embodiment of the invention the charging electrode maycomprise an arcuate plate electrode composed of a sintered metal.

FIGURE 1 is a perspective view of a suitable embodiment employing analuminum foil electrode for the corona treatment of thermoplastic filmsuch as polyethylene or polypropylene.

FIGURE 2 is a cross section of a sintered arcuate 3,345,48fi PatentedOct. 10, 1967 plate electrode employed in lieu of the aluminum foilelectrode shown in FIGURE 1.

The invention is best described with reference to the drawings. In thistype of apparatus, which is shown as enclosed within a framework 2 butwhich may employ any suitable frame structure, is a grounded steelcylindrical electrode 4 mounted on a shaft 6 driven by any suitabledriving means such as motor 8. The opposite end may be mounted onto theframework by any suitable means such as a journal box 10. The exteriorsurface of the grounded electrode 4 is preferably, but not necessarily,covered with an insulating substance 12, such as rubber. Although thedirection of rotation is not important, the grounded electrode is shownas rotating in the clockwise position looking from the motor end of theshaft. The film 14 enters the frame 2 through a slot 3, passes underguide roller 5, over electrode 4 and under guide roller 7.

Mounted directly above the ground electrode is a thin, flexible,electrically conductive smooth sheet-like material, such as aluminumfoil, which may be attached to any suitably supporting means 9 or to ablade electrode (not shown). The aluminum foil may be attached at anysuitable location on the foil such as at the middle portion or at anyedge, for example, the posterior edge relative to the movement of thefilm. However, the foil can be supported at only one point or in anyplane transverse the moving web. Upon the application of a potentialacross the electrodes, the aluminum foil is attracted to the groundedelectrode but does not contact the same and is self supported at adistance which is conductive to the formation of a corona between thealuminum foil and the grounded electrode. The mechanism for thisphenomena is not understood but may be related to electrostatic forces.

Although the invention is discussed herein primarily with reference tothe employment of a thin sheet of aluminum foil, it is to be understoodthat any electrically conductive material may be employed, for examplethin flexible sheets of copper, iron, steel, brass, and the like.However, it is also within the scope of the invention to employ awrinkled sheet of foil. Preferably, the foil electrode has a thicknessof from 0.1 to 1.0 mil. However, the foil thickness is not necessarilylimitative so long as the sheet is flexible enough to permit theformation of a substantially uniform corona. The mechanism for thisphenomenon is not known but apparently is related to the vibration ofthe sheet. The width of the electrode should be suflicient to cover thefilm and the length is preferably at least 1 inch, more preferably 1inch to /2 the electrode circumference. The distance between thealuminum foil and the grounded electrode during treatment is generallyless than inch, preferably in the range of from to 3 inch. Prior totreatment the foil should be positioned about to 1 inch from thegrounded electrode. The aluminum foil electrode 13 is connected to apower source (not shown) through electrical conduit 15. The groundedelectrode is grounded through conduit 17.

The sintered electrode 20 is shown in cross section in FIGURE 2 in lieuof aluminum .foil electrode 13 shown in FIGURE 1. The sintered electrodemay be supported by any suitable means. However, in contrast to thealuminum foil electrode, the sintered electrode is relatively thick,does not vibrate and does not seek the corona-producing distance. Sincethe sintered electrode is a porous metallic material, gases may beintroduced through the electrode into the space between the electrodes.Apparent ly the sintered electrode provides for a large number of peaksor points which uniformly distribute the corona 22 throughout the space.

Although the invention has been described with reference to theemployment of the aluminum foil and the sintered metal electrodes as thecharging electrodes, it is within the scope of the invention to employthese electrodes as the discharging electrodes or to employ them as bothelectrodes or in any combination thereof.

The invention is not limited to the particular type of product beingtreated by the corona, however, the invention is particularly applicableto the treatment of filmlike materials including all organicthermoplastic and thermosetting resins such as, but not limited to,polyolefins, including polyethylene, polypropylene, polybutene-l and thelike, polyvinyls, vinyl-chloride copolymers, polyamides, includingnylon, and the like. The term polymer as employed herein includeshomopolymers, copolymers, te-rpolymers, block copolymers, laminates andthe like. The film may be irradiated and/ or molecularly oriented. Thefilm thickness is not critical but is preferably between 0.1 and 20mils.

referably the frequency is in the range of 10 to 1000 -kc., morepreferably 100 to 300 -kc. The voltage and current are variable over awide range and are sufiicient to provide a corona discharge for the gapemployed between the film and the charging electrode. Suitable voltagesare in excess of 100, preferably 500 to 10,000 kc. Suitable currents arein excess of 0.7 amps, preferably 1 to 1.5 amps.

The lower electrode may be covered by any suitable buffer dielectricmaterial as hereinbefore described.

The invention is best described with reference to the followingexamples:

Example I Using a high frequency high voltage generator Model HFSG-Z asthe power source, an aluminum foil electrode measuring 18%" Wide x 5"long X 0.5 mil thick was separated from a inch thick rubber coveredground roll by a & fiberglass screen. A .75 mil polypropylene (Profax6420) film and a .75 mil polyethylene (Alathon 1412) film passed betweenthe rubber covered ground roll and the fiberglass screen.

Current amps Watts 1100 Film speed -f.p.m 151 Resalt.--There was nobreakdown of film and a uniform oxidation pattern was produced over thesurface of electrode indicating a uniform corona (oxidation pattern asleft by corona).

Example 11 Using a high frequency high voltage generator, Model HFGS-2,a sintered stainless steel electrode, measuring 18% wide x 5" long x /sthick, was spaced approximately from the A rubber covered ground rollwhile the film (see Example I) was passed between the electrodes.

Current amps 1.3 Watts 1100 Film speed f.p.m 150 Results.N breakdown offilm and a uniform oxidation pattern over surf-ace of electrode.

Example 111 Using a high frequency high voltage generator, Model HFSG-Z,a polished aluminum electrode measuring 18% wide x long x A" thick wasspaced approximately from the A1" rubber covered ground roll while thefilm (see Example I) was passed between the electrodes.

Current amps 1.4 Watts 1100 Film speed f.p.m 150 Resalts.-Some scorchingof the film and a non-uniform oxidation pattern over the surface ofelectrode thus indicating that a non-uniform corona was produced.

4 Example IV Using a high frequency high voltage generator, ModelJFSG-Z, a steel blade electrode measuring 18%" wide x thick was spacedfrom a A" rubber covered ground roll while the film (see Example I) waspassed between the electrodes.

Current amps .9 Watts 300 Film speed f.p.m

Resalts.-There was much scorching of film.

While certain examples, structures, composition and process steps havebeen described for purposes of illustration, the invention is notlimited to these. Variation and modification within the scope of thedisclosure and the claims can readily be effected by those skilled inthe art.

I claim:

1. Apparatus for corona treating film comprising, in combination:

(a) at least two spaced apart electrodes chargeable to oppositepolarities,

(b) means for applying a high value alternating potential across saidelectrodes sufiicient to form a corona therebetween, and

(c) .at least one of said electrodes comprising a flexible metal foilelectrode free to adjust itself to a substantially constant coronaproducing distance from the opposite electrode, and

(d) means for passing said film between said electrodes.

2. Apparatus for corona treating film comprising, in

combination:

(a) at least two spaced apart electrodes chargeable to oppositepolarities,

(b) means for applying a high value alternating potential across saidelectrodes suflicient to form a corona between said electrodes,

(c) at least one of said electrodes comprising a flexible metal foilelectrode minimumly supported, the remainder of the flexible electrodebeing unsupported and free to adjust itself to a corona producingdistance upon the application of a potential across said electrodes, and

((1) means for passing said film between said electrodes.

3. The apparatus of claim 2 wherein said flexible electrode is aluminumfoil.

4. The apparatus of claim 2 wherein said flexible electrode is supportedby attaching an edge of said flexible electrode to a blade electrode.

5. Apparatus for corona treating film comprising, in combination:

(a) a pair of spaced apart electrodes chargeable to opposite polarities,

(b) means for continuously passing a film between said electrodes,

(c) means for applying a high value alternating potential across saidelectrodes,

(d) the discharging elect-rode comprising a grounded metal roller,

(e) the charging electrode comprising a thin, flexible sheet of aluminumfoil fixedly secured above said film the remainder of the foil beingunsupported and free to adapt itself to a corona producing distance uponthe application of a potential across said electrodes.

References Cited UNITED STATES PATENTS 2,969,463 1/1961 McDonald 204-1 65 3,068,510 12/1962 Coleman 20f1:-165

JOHN H. MACK, Primary Examiner.

ROBERT K. MIHALEK, Examiner.

1. APPARATUS FOR CORONA TREATING FILM COMPRISING, IN COMBINATION: (A) ATLEAST TWO SPACED APART ELECTRODES CHARGEABLE TO OPPOSITE POLARITIES, (B)MEANS FOR APPLYING A HIGH VALUE ALTERNATING POTENTIAL ACROSS SAIDELECTRODES SUFFICIENT TO FORM A CORONA THEREBETWEEN, AND (C) AT LEASTONE OF SAID ELECTRODES COMPRISING A FLEXIBLE METAL FOIL ELECTRODE FREETO ADJUST ITSELF TO A SUBSTANTIALLY CONSTANT CORONA PRODUCING DISTANCEFROM THE OPPOSITE ELECTRODE, AND (D) MEANS FOR PASSING SAID FILM BETWEENSAID ELECTRODES.